Would the Half-Elven vanish if they wore an elven ring?

[Alfirin]

That does make some sense, especially how it applies to later generations i.e. that it seems as if, when an elf (or half elf) chooses mortality that applies not only to themselves, but all thier descendents. I am thinking here of Elros. When he chooses the mortal life (i.e. to become Tar-Minyutar, first king of Numenor) all of his descendents also become fully mortal. Elrond's line keep the choice, Elros's loses it. So the system is sort of skewed in favor of choosing mortal (each individual who makes the choice is equally free to make either, but a choice of mortality also takes the right to choose away from any children you may have, while choosing immortal doesn't) I may be wrong (after all we are looking at a very small number of examples, so the results may be a little biased) but at least that is how it seems to me (and if the argument is that Elros's children don't get the choice becuse they are 3/4 mortal, I would argue that by that logic, Elrond's should also not have gotten it; as 3/4 elf, they should have HAD to be immortal)

[Morthoron]

Originally Posted by Alfirin

That does make some sense, especially how it applies to later generations i.e. that it seems as if, when an elf (or half elf) chooses mortality that applies not only to themselves, but all thier descendents. I am thinking here of Elros. When he chooses the mortal life (i.e. to become Tar-Minyutar, first king of Numenor) all of his descendents also become fully mortal. Elrond's line keep the choice, Elros's loses it. So the system is sort of skewed in favor of choosing mortal (each individual who makes the choice is equally free to make either, but a choice of mortality also takes the right to choose away from any children you may have, while choosing immortal doesn't) I may be wrong (after all we are looking at a very small number of examples, so the results may be a little biased) but at least that is how it seems to me (and if the argument is that Elros's children don't get the choice becuse they are 3/4 mortal, I would argue that by that logic, Elrond's should also not have gotten it; as 3/4 elf, they should have HAD to be immortal)

Perhaps he was only fractionally invisible, like a baby toe or pinky finger.

[Findegil]

We do not have to extract the rules from the view examples we have. Tolkien reported the decision about the half-elves very accurately [The history of Middle-earth; volume V: The Lost Road and Other Writings: Language and Legend before The Lord of the Rings; part two:Valinor and Middle-Earth before The Lord of the Rings; chapter VI: Quenta Silmarillion; sub-chapter: The conclusion of the Quenta Silmarillion; paragraph 9]:

Then Manwe gave judgement and he said: 'To Earendel I remit the ban, and the peril that he took upon himself out of love for the Two Kindreds shall not fall on him; neither shall it fall upon Elwing who entered into peril for love of Earendel: save only in this: they shall not ever walk again among Elves or Men in the Outer Lands. Now all those who have the blood of mortal Men, in whatever part, great or small, are mortal, unless other doom be granted to them; but in this matter the power of doom is given to me. This is my decree: to Earendel and to Elwing and to their sons shall be given leave each to choose freely under which kindred they shall be judged.'

The children of Elrond would have had no choice if it would not have been granted to them by Manwe. It is an interesting question how they and their family did get that information, but it is reported by Elrond to Aragorn in the case of Arwen, so they did know.

Anyhow this does not say anything about Dior. But I agree with Puddleglum. Dior was the son of Beren and Luthien while both were mortals, so he was a mortal. That he might not have been happy with that is clear, but to marry an elven woman was his choice as is it was the choice of Imrazor who married Mithrelas. Neither of these couples could change the doom of separation as long as Ea my last or even longer (practically for ever).

About Gandalf confusing the 3 with the other great Rings in his explanation to Frodo: Is it at this point necessary for Gandalf to be absolutely precise about this feature? I don't think so. He is to explain Frodo which of the great rings the one Fordo has is. The 3 are accounted for, but he is not permitted to speak about them. And Gandalf and Frod do know that Frodos ring did make the wearer (at least a mortal) invisible. The important point is to warn Frodo about the 'side'-effect of using the ring often (fading to permanent invisibility). It would be completely useless to tell Frodo that he would not be in that danger would his ring be one of the 3.
Since the 3 are accounted for, the effect that the ring did confer invisibility to Fordo and Bilbo is an evidence that it is one of the great and that is a farther point why Gandalf does mention it here. After giving farther evidence Gandalf doe then make the final test and proves the identity of Fordos ring. So it is at this point (and also later at the council of Elrond) unnecessary to explicit explain the whereabouts and exact properties of all the other 18 great rings. (BTW this is a petty, as I would have like a full account of where and when the 4 dwarven rings that Sauron had not collected were destroyed and how the Wise did know.)

Back to the original question: Would Half-Elves become invisible wearing one of the Rings that conferred invisibility?
I think it depends. Elrond: No. When he had a chance to lay hand on one of these rings he had already chosen to be an elf. So by all means he was.
Arwen: I think her choice was made effective when she married Aragorn. Otherwise Elrond delaying of the marriage would be useless. So I would say up to that point she had the life and vigor of the Elves and therefore would not become invisible. (If Elrond would have allowed her marriage earlier, I would think she would have become invisible wearing one of the rings in question, but that chance never came.)
Elladan and Elrohir: Since we know very little about the doom spoken about them we can only guess by the similarity with Arwen. So I think they wouldn't become invisible, since we are told in one of the letters that they probably delayed their choice even after Elrond had left Middle-Earth. So as long as the rings were functional they still had the life and vigor of the elves.
Earendil and Elwing: They did not have any chance to get hold of one of the rings at all. (But if there would have been a chance, I think they would not become invisible.)
(If we ignore for the sake of speculation the time line: Elwing, Earendil, Elrond and Elros before their choice: Difficult, but I would again go with the similarity of Arwen => as long as the doom is unclear the life and vigor of the elves is granted and so no invisibility.
Elros after his choice: See again the similarity with Arwen. Once he was a mortal he would become invisible wearing one of the rings in question.)

Respectfuly
Findegil

[Alfirin]

Originally Posted by Findegil

About Gandalf confusing the 3 with the other great Rings in his explanation to Frodo: Is it at this point necessary for Gandalf to be absolutely precise about this feature? I don't think so. He is to explain Frodo which of the great rings the one Fordo has is. The 3 are accounted for, but he is not permitted to speak about them. And Gandalf and Frod do know that Frodos ring did make the wearer (at least a mortal) invisible. The important point is to warn Frodo about the 'side'-effect of using the ring often (fading to permanent invisibility). It would be completely useless to tell Frodo that he would not be in that danger would his ring be one of the 3.
Since the 3 are accounted for, the effect that the ring did confer invisibility to Fordo and Bilbo is an evidence that it is one of the great and that is a farther point why Gandalf does mention it here. After giving farther evidence Gandalf doe then make the final test and proves the identity of Fordos ring. So it is at this point (and also later at the council of Elrond) unnecessary to explicit explain the whereabouts and exact properties of all the other 18 great rings. (BTW this is a petty, as I would have like a full account of where and when the 4 dwarven rings that Sauron had not collected were destroyed and how the Wise did know.)

Findegil

It is genrally accepted that the 4 other dwarven rings were melted by Dragon fire (usually when the dwarf wearing them was eaten by said dragon) As for how the Wise might know, Gandalf isn't exactly on bad terms with the Dwarves (he's not exactly on good terms either, but they certainly don't hate him. Since each dwarf who posessed a ring wound up becoming very rich and powerful, accounts of thier passing would presumably have been kept by the dwarves (not to mentioned that "so-and-so was devoured by a dragon" is certianly the kind of story that would be passed on. The Dwarves may have kept records of who recived the rings (given how resistant they were to the corrupting influences of the rings, I do not know if dwaves who had one would have made a point of hiding the fact). If nothing else, whatever descendenct each of those dwarves had would presumably know thier family had one, and that after the dragon had done what it did, the ring was not there (given how valuable they are, I would not put it past a decendent to attempt to dig the ring out of the stomach (if the dragon in question was slain after eating the given relative, or checking it's "leavings" (if it left any))
What I find odd about Gandalf's speech is that it seems to contain information he probably couldn't know. Gandalf mentions that starting from good intentions delays the effect of the rings corruption. How does he know that? The Dwarves are practically uncorrupted anyway, so not from them, The One has been lost for ages, and Bilbo/Frodo quite simply havent had the ring long enough for the full effect to have take hold (unless in the hands of someone evil the transformation is truly rapid, to the point where they in fact reach this weary of life at a time that would in fact fall within thier normal lifespan (i.e. they do not in fact gain ANYTHING from the ring) so he must be referring to the Nazgul, and insinuating that some of them were not "evil" people before they took the ring. How has he found that out? By the time the Ishtari arrive, all of the Nazgul have become thoroughly "wraith-ified" so it's not like Gandalf could have ever known any of them before. How could any of them. The elves did not have all that much contact with men (with a few notable exceptions) and men seem to have been far more secretive about thier ring posession. About the only method I can think of is that someone from Numenor (where Sauron handed out the last 3 rings knew someone who took one, and chronicled what happened to him (or I suppose, even a record from one of the holders themselves, when they were young and stll not taken over) and that those records made it to either the Gondor Archives (where Saruman or Gandalf could have read them) or to an elf ear (given that it was sort of his brother's kingdom, I can imagine Elrond would at least listen to the goings on of Numenor when someone had a tale to tell, estrangement or no.)